Hydrogen/air fuel cells (H/AFCs) have enormous potential as a replacement for batteries. Because they can operate on very energy-dense fuels, fuel cell-based power supplies offer high energy-to-weight ratios compared with even state-of-the-art batteries. Fuel cells are of particular interest to the military, where significant efforts are being made to reduce the weight of power supplies that soldiers must carry to support high-tech, field-portable equipment. There is also considerable potential for utilizing fuel cell-based power supplies for commercial applications, particularly where small size and low weight are desirable.
Functionally, fuel cells generate electricity by reacting hydrogen with oxygen to produce water. Since oxygen can typically be obtained from the ambient atmosphere, only a source of hydrogen must be provided to operate a fuel cell. Merely providing compressed hydrogen is not always a viable option, because of the substantial volume that even a highly compressed gas occupies. Liquid hydrogen, which occupies less volume, is a cryogenic liquid, and a significant amount of energy is required to maintain the extremely low temperatures required to maintain it as a liquid.
Several alternative approaches are available. These alternatives include hydrocarbon and methanol fuel reforming, hydrogen absorption into metal hydrides, hydrogen-generating chemical reactions, and ammonia decomposition. The ammonia decomposition reaction can be represented as follows:2NH3+ENERGY→N2+3H2 
Generating hydrogen from ammonia is particularly attractive because the usable hydrogen yield per kilogram of ammonia is relatively high, and the decomposition of ammonia to generate hydrogen is a well understood and relatively straightforward reaction. Because ammonia is readily available and relatively inexpensive, and because it offers a substantial yield of hydrogen, it is a desideratum to develop an efficient apparatus for processing ammonia to generate hydrogen for fuel cells and other purposes.
To compete with battery-based power supplies, such an H/AFC apparatus needs to be compact and reliable. It is a further desideratum to develop a portable hydrogen supply with a volume less than 1 liter and a mass less than 1 kg that can produces up to 50 watts of electrical power, with a total energy output of 1 kWh. Commercially available metal hydride storage cylinders are available in 920 gm cylinders that contain the equivalent of 100 W-h of hydrogen; thus, a total energy output of 1 kWh represents an order of magnitude increase in energy density over commercially available apparatuses.
One of the challenges of utilizing ammonia to produce hydrogen for a fuel cell is that H/AFCs do not tolerate ammonia in the hydrogen feed gas, so the trace amounts of ammonia in the H2/N2 gas mixture produced by an ammonia cracker must be removed before the mixture is supplied to a fuel cell. Commercially available ammonia adsorbents (e.g., acid-impregnated carbon) can be used for this purpose, but the required adsorbent mass of such materials can be prohibitively large, if the ammonia-cracking reactor does not provide high conversion efficiency.
Employing a relatively high reaction temperature (over 850° C.) reduces the amount of ammonia in the H2/N2 product, and the amount of adsorbent required. However, using such a high reaction temperature imposes significant design challenges. While the mass of adsorbent required is reduced, high temperature reactors must be fabricated of high temperature refractory metals, such as Inconel and molybdenum. These materials often require complex fabrication techniques, such as diffusion bonding, as opposed to more conventional brazing or laser welding techniques that can be used with more conventional materials, such as stainless steel or titanium.
Furthermore, for any given design, heat loss to the ambient environment from a reactor is increased as the reactor temperature is increased. Increasing the reactor temperature reduces overall energy efficiency or results in an increase in the apparatus size and weight due to the requirement for additional insulation.
Furthermore, the hydrogen generation reactor employs a catalyst. Catalysts have a minimum temperature, referred to as the light-off temperature, at which the catalyst facilitates the ammonia decomposition reaction, and a maximum operating temperature, which is generally a function of the catalyst and its support matrix, if any. Catalytic materials are often dispersed in a support matrix. For example, it is very common to distribute catalytic material on an alumina support. Such supports have a maximum allowed operating temperature. For example, in excess of 850° C., an alumina support can become sintered, (i.e., the alumina support components begin to fuse together). At that point, the efficiency of the catalyst drops dramatically. Consequently, temperatures in excess of 850° C. are incompatible with many types of potential catalysts, particularly supported catalysts.
By lowering the reactor temperature and accepting an increase in the levels of residual ammonia in the H2/N2 product, the design constraints on the apparatus are reduced. Conventional materials and fabrication techniques can be employed and a greater variety of catalysts and catalyst supports can be used. However, in prior art designs, a relatively large volume of adsorbent is required to remove the residual ammonia, significantly increasing the mass of the low temperature hydrogen generating apparatus.
Because creating a compact hydrogen generating apparatus is critical to increasing the utilization of fuel cell technology, decreasing the mass of adsorbent required to enable a relatively low reactor temperature apparatus to be used is critical to minimizing the size of such a compact apparatus. For example, in the target apparatus capable of producing 50 Watts power and 1 kWh of energy, and having a mass of 1 kg, if the ammonia reactor runs at a 99.0% conversion, a total of 3.33 g of ammonia must be removed from the H2/N2 gas mixture exiting the reactor. Commercially available ammonia adsorbents may collect only up to about 1% by weight of ammonia (given a relatively low concentration of ammonia in a gas stream) in the presence of traces of water (ppm levels) that is typically present in commercial grade ammonia, so about 333 g of adsorbent would be required for an ammonia-cracking reactor running at 99.0% conversion efficiency. Thus, the adsorbent mass alone represents one-third of the target mass, leaving too little mass available for the other elements of the hydrogen generation apparatus. Consequently, it is also a desideratum to develop an ammonia-based hydrogen generation apparatus that operates at temperatures less than 850° C.
A further challenge in providing a compact ammonia-based hydrogen generating apparatus for use with fuel cells and other applications is in selecting a reactor that achieves the desired compactness. One factor to be considered when evaluating a reactor is the residence time required to achieve desired conversion efficiency. Longer residence times require a larger reactor volume. To achieve a compact ammonia-based hydrogen generating apparatus, very short residence times are required to enable very small volume reactors to be employed. As the size of the reactor increases, so will its weight.
Conventional, large-scale hydrogen generation reactors often use packed-beds in which ammonia is passed through a heated vessel containing millimeter-sized pellets of catalyst materials. In many cases, the actual reaction rate in these reactors is considerably slower than the theoretically possible reaction rate (i.e., the rate expected based upon the intrinsic reaction kinetics) because of heat- and mass-transfer resistances. Therefore, it is also a desideratum to provide a reactor whose dimensions favor rapid heat and mass transfer, and short residence times.
Clearly it would be desirable to provide a compact ammonia-based hydrogen generating apparatus for use with fuel cells and other applications that operates at a relatively low temperature (i.e., from about 550° C. to about 650° C.), yet which does not require a significant volume of adsorbent to be employed to remove residual ammonia from the H2/N2 product, and which avoids the use of a packed bed reactor. The present invention satisfies these and other needs, and provides further related advantages.